The purpose of this investigation is to study rheological changes in* hemorrhagic and other forms of shock and to elucidate the interrelation between blood rheology and oxygen transport. Cardiac output (dye dilution method), coronary blood flow (Xe 133 washout), total body oxygen consumption and myocardial oxygen consumption will be determined in the pentobarbitalized and unanesthetized dogs. The hematocrit of the animal will be adjusted to various levels (20-60 percent) by isovolumetric exchange with plasma or packed red cells. Blood viscosity will be determined in a coaxial cylinder viscometer over a wide range of shear rates. The results on oxygen consumption and blood flow will be correlated with the hematocrit and blood rheology, with the aim of establishing the optimum hematocrit for oxygen delivery, especially to the myocardium. The correlation of in vitro blood rheology with in vivo hemodynamics will also be carried out in patients in low flow states due to surgical operation or other critical illnesses.